Converting from previous editions and eyeballing monsters

Monster Statistics. For first and second edition, you can use the shorthand monster statistics in the adventure. In
these adventures, monsters are often more numerous, but they typically deal lower damage and have fewer hit points than their fifth edition counterparts. Make the following changes to those first- and second-edition monsters:

Armor Class equals 19 minus the creature’s AC, up to AC 22.

Attack roll modifiers are HD/2 + 2, up to +12.

Saving throw DCs are 8 + HD/2, up to 20.

If a creature has to make a check or saving throw, and should be good at the roll, use the creature’s HD/2 + 2 as a bonus on the roll. Otherwise, use no modifier, or use a penalty to reflect something the creature should be bad at.

I don’t love these rules. They don’t go far enough in giving me what I need to build new monsters, and don’t help me estimate much.

Let’s renormalize around hit points instead. The numbers below are baseline-y, so definitely do deviate from them.

Calculate the previous-edition HD and therefore hp (=HD*5, for simplicity).

Our 5e creature has:

The same number of hit points.

Challenge of HD/8 (=HP/40).

Armor Class of 12 + HD/4, max 22.

Attack Bonus of +2 + HD/4, max +12.

Saving throw Difficulty Class of 10 + HD/4, max of 20.

Saving throw and check modifiers of + HD/4 (max of +10), with +2 to a couple of good ones and a penalty from -5 (quite weak!) to +0 to the others.

Spells track poorly to challenge, but most monsters cast spells whose level is around 2/3 their challenge — meaning that you should aim for spells of level approximately HD/12. This number is conservative; remember the pixie!

Remember that these numbers are baselines, and track (very!) imperfectly to the DMG: rob it of 2 points of AC for a 50% boost in HP, give it magic resistance and mundane weaponry resistance in exchange for 50% of its hp, 3d6 damage instead of 2d10, and so forth.

“Hey, these rules make a 2e ogre as tough as a 5e orc… 😦“. Yeah, they do, because a 2e ogre is as tough as a 5e orc. A 4HD monster like the 2e ogre has 20 hp, AC 13, +3 attack, and deals 1d6+2 damage on a hit, and has an estimated CR 1/2. That’s self-consistent by the numbers, but 5e’s scale is actually much steeper than 2e’s, here. If we use these rules to project back into previous D&Ds, the 5e goblin is a 1+1HD monster, the 5e orc a 3HD monster, and the 5e ogre a whopping 12HD monster.

If you’re white-room converting monsters for worldbuilding (which is what I’m doing) you may need to triple 2e hit dice to retain their approximate position in the world through a 5e filter. If you do that on the fly while converting an adventure, you’d need to remember to reduce the number of monsters to a third: earlier adventures increased difficulty by increasing headcount.